This invention relates to carburetors for internal combustion engines and in particular to carburetors for small internal combustion engines such as are used in lawnmowers, snowblowers, generators, pumps and the like.
Prior art carburetors generally include a die-cast body made, for instance, from cast aluminum or zinc to which a fuel bowl assembly is secured by means of threaded fasteners. A disadvantage of this prior art carburetor construction is the high cost of manufacturing the die-cast aluminum body. Cost is incurred not only by casting the body but also by machining of the cast body to provide numerous passages and apertures. Another disadvantage of such prior art die-cast aluminum bodies is that die cast aluminum is often porous so that such die-cast aluminum bodies must be impregnated with a special sealing material. Other die-casting materials have also been used, such as, for instance zinc which is less porous than aluminum. However, zinc is both higher in weight and cost than aluminum and is therefore not a preferred material since it is important to make small internal combustion engines light in weight, particularly those engines which are used in hand-held or easily maneuvered equipment, such as leaf blowers, lightweight snowblowers, lawnmower and the like.
The fuel bowls of prior art carburetor assemblies have generally been made of either stamped steel or aluminum. Completely molded plastic prior art carburetors have also been provided in an attempt to reduce the machining and the overall number of separate components required for a carburetor. By manufacturing the entire carburetor from plastic, many of the details which would normally be machined may be molded in. However, it is difficult to mold certain of the orifices and other features which must be held to required close tolerances such as, for instance, .+-.0.001 of an inch, especially those with spans of 0.250 inch and up.
Two items which are particularly difficult to mold in plastic carburetors are the throttle bore and the throttle shaft bore. Both of these bores must be held to very tight tolerances and their alignment in the carburetor body is critical. Good performance of a carburetor requires a true throttle bore, especially in a full progression carburetor. Even if close tolerances can be held during the plastic molding process, in time, after the plastic structure has been subjected to thermal cycling and/or stress under load, the plastic material tends to deform due to plastic creep and the tolerance limits are therefore exceeded. Manufacturers have attempted to avoid this problem by eliminating certain functions from the carburetor such as for instance an idle system, thereby both limiting performance capabilities and avoiding the need for precision bores.
In some plastic carburetor structures, the throttle bore, throttle shaft bore and idle progression holes are machined in an aluminum portion of the carburetor in order to ensure close tolerances. In another attempt to improve the performance of molded plastic carburetors, high quality glass reinforced plastic or mineral filled plastic materials have been used. However, such filler materials make drilling and machining of the plastic carburetor very difficult. Furthermore the cost of high quality plastic can be as much as the cost of aluminum.
Still another problem with prior art plastic carburetors has been that the performance of some plastics will deteriorate by contact with gasoline, gasoline/alcohol blended fuels, and especially decomposing gasoline which generates acids and peroxides
Other problems with prior art carburetors have been the cost of assembly which generally involves numerous gaskets and 0-rings, threaded fasteners, and press fit components. Another major problem has been difficulty of servicing the carburetor. Prior art carburetors generally needed to be completely disassembled from the engine in order to be serviced. In order to reassemble the carburetor to the engine, all linkages had to be reconnected and readjusted, all of which was time consuming.
It is, therefore, desired to provide a carburetor for small internal combustion engines which is low in cost, has excellent performance, is simple to assemble and is easy to service.